1. Field of the Invention
This invention relates generally to the purification of germanium hydrides through selective absorption, partial condensation, freezing, phase separation and adsorption of impurities, followed by fractionating the purified gas mixture; and more specifically to such sequential operations that are designed for removing water and carbon dioxide from mixtures of hydrogen and germanium hydrides before fractionating the dry and de-carbonated gas mixture into substantially pure components.
2. Description of the Related Art
In the last half-century, the electronic materials industry has developed effective techniques of chemical vapor deposition, which include metalorganic vapor phase epitaxy, hydride vapor phase epitaxy, metalorganic molecular beam epitaxy and atomic layer deposition. These technologies consume metalloid compounds that include the ultra pure hydrides of silicon, germanium, phosphorous and boron in an ambient gas such as hydrogen, to make diverse structures that include molecular units, crystals, surfaces, thin films and alloys such as silicon-germanium. Germanium based structures find wide applications in high-performance microelectronic devices, such as diodes, transistors, detectors, thin-film photovoltaic cells and the like. By virtue of lower deposition temperatures and ease of treating decomposition by-product hydrogen in vent systems, germanium hydrides present a more compelling business case to foundry users than the halides of germanium. Not surprisingly, recent years have witnessed a growing demand for ultra pure germane and digermane, with 10 ppm or less of impurities, driven mainly by consumption in silicon-germanium alloys for microelectronic devices and amorphous thin films for multi-function photovoltaic cells. On the one hand, silicon-germanium alloys find applications in heterojunction bipolar transistors or as strain-inducing layers for complementary metal oxide semiconductor transistors in integrated circuits. This silicon-germanium technology enables higher processor speeds and a flexibility to tune band gaps in low-cost electronic processors to meet the specific bandwidth needs of high frequency optical networking, wireless and other communication applications with greater efficacy than chips made with silicon only. On the other hand, germanium is an invaluable component of multi-junction photovoltaic cells, each comprising an ordered assembly of different semiconducting thin-film junctions, each junction generating a photovoltaic current in response to specific frequencies of incident sun light. The most used thin-film material silicon, for example generates a photovoltaic current from incident sun light over a comparatively small portion of the solar spectrum; its lowest absorption is in the infrared, which is where germanium with its small band gap provides a stronger photovoltaic response, converting incident infrared photons into electric current. In addition to silicon and germanium layers, multi-junction photovoltaic cells have evolved to include layers of gallium arsenide for converting blue light and indium phosphide for converting incident ultraviolet frequencies into electric currents. A multi-junction assembly of these material layers provides a very large capture cross-section for incident sun light, leading to photovoltaic cells with higher conversion efficiencies or base currents than are currently obtainable from cells based on silicon only.
Methods for synthesizing germanium hydride gases are long known in the art, including general considerations of raw materials, their physical and chemical properties and the impact of reactants, pH and temperature on yields. The methods include the chemical reduction of germanium oxide in acidic media or germanium halide in alkaline media to produce germanium hydrides and hydrogen gas mixtures. Typical reducing agents include the hydrides of lithium, sodium and magnesium, the borohydrides of lithium, sodium and potassium and as well as the aluminum hydrides of lithium and sodium. Another chemical method involves the reduction of germanium halide in a heated solution of a reducing agent in tetrahydrofuran solvent. Illustrative of the early chemical syntheses prior art for producing germanium hydrides are those of T. S. Piper et al, (1957); Macklen (1959); T. N. Srivastava et al (1962); W. L. Jolly (1961) and J. E. Drake et al (1962). Of the early known art, Drake J. E. et al (1962) obtained about 70% yields of germanium hydride by acidifying alkaline solutions of alkali metal borohydride containing germanium oxide in various concentrations with or without a polyglycol additive. The reagents used (germanium oxide-GeO2, potassium hydroxide—KOH, sulfuric acid-H2SO4, sodium borohydride—NaBH4 and water) were further investigated by Russotti in U.S. Pat. No. 4,668,502 and shown to provide yields in excess of 96% germanium hydride for feed ratios of 6:1 NaBH4 to 0.13M GeO2 and 1:2 GeO2 to 1.5-3.0M H2SO4. Russotti also teaches that side reactions occur, especially with the use of more concentrated acid at warmer temperatures to produce digermanium hexahydride (Ge2H6). The presence of excess acid, for example H2SO4, in aqueous solution is well-known in the art to catalyze the hydrolysis of excess NaBH4 into hydrogen and sodium tetrahydroxoborate—Na[B(OH)4]. The hydrolysis of NaBH4, represented in scientific notation as NaBH4+4H2O ⇄ Na[B(OH)4]+4H2 is spontaneous and exothermic with a theoretical heat output of −245 kJ/mol. Typical studies of the kinetics of NaBH4 hydrolysis are those of Gardiner et al (1965), Wang et al (1972), Kreevoy et al (1972). The overall heat output in underlying reactions, if uncontrolled, is sufficient to raise the temperature of the aqueous solution, thereby raising the vapor pressure of water in product gases and as well as that of other atmospheric gases dissolved in aqueous media. Such dissolved gases include but are not limited to carbon dioxide, nitrogen, oxygen and argon. These solution gases, though small in quantity, add to hydrogen and germanium hydrides in the product gas mixture bubbled off aqueous reactions in chemical reduction methods employing aqueous reactants. U.S. Pat. No. 5,158,658 to Ayers teaches an electrochemical method for synthesizing germanium hydride wherein the product is generated at a germanium cathode of a 1N NaOH electrolyte with a cadmium or molybdenum anode. The exemplified yield of 30% GeH4 at the cathode is clearly lower than can be obtained from the basic chemical reduction method of Drake (1962).
Regardless of synthesis method used, the hydride gas mixture must be refined to obtain germanium hydrides in ultra pure form, typically over 99.999% pure or five 9s, which are generally stated as “grade 5.0” to meet the specification for electronic device manufacturing. Techniques for selectively removing moisture and carbon dioxide from gas streams include but are not limited to physical absorption, chemical absorption, partial condensation and phase separation, selective freezing in reversing heat exchangers, selective permeation, adsorption and distillation.
Since the teachings in U.S. Pat. Nos. 2,882,243 and 2,882,244 to Robert M. Milton that electrostatically bound and charge compensating cations in aluminosilicate crystals are substitutable to obtain evenly porous and internally charged media that have molecular sieving properties, application of zeolitic molecular sieves or zeolites in gas purification, particularly as dewatering and de-carbonating media for a wide range of fluids even at very low concentrations, are now taught in the art (Scott M. A. et al, Handbook of Zeolite Science and Technology, Dekker, 2003) and exemplified in commercial practice on a world wide scale. Examples include adsorption systems for removing water and carbon dioxide from (a) natural gas to raise its calorific value, (b) commercial hydrogen made by steam-methane reforming, (c) compressed air upstream of cryogenic distillation in industrial air separation, (d) krypton, xenon and neon enriched gas streams recovered from air following the catalytic conversion of trace hydrocarbons and oxygen impurities into carbon dioxide and water and from (e) intermediates of some flavor-organic compounds. Illustrative examples describing the use of zeolites to remove contaminants, notably water and carbon dioxide from hydrogen gas enriched streams to obtain high purity hydrogen are those described in U.S. Pat. Nos. 3,788,037; 3,102,013; 3,176,444; 3,221,476; 3,323,288; 3,430,418; 3,619,984; 3,720,042; 3,751,878; 3,957,463; 4,077,779. These methods teach the use of step changes in temperature, in pressure or in the concentration of gas flowing through the media to accomplish adsorption and desorption of water and/or carbon dioxide molecules from zeolites.
The most common group of zeolites is type A (3A, 4A and 5A), having the same crystalline structure but different cations on unit surfaces which produce different pore sizes. At ambient temperature and lower, the affinity for water and carbon dioxide molecules by molecular sieves type A is so strong that temperatures in excess of 300° C. may be needed to desorb water molecules fully. The desorption sequence known as regeneration enables adsorbate (water and carbon dioxide) molecules to be thermally driven off the zeolite surface, rendering the latter re-usable as a purification medium at lower temperatures. While the strong affinity for water and other polar molecules renders molecular sieves costly in thermal energy and regeneration time, synthetic zeolites, especially types A and 13X in 4-8 mesh sizes are the economic method of choice for removing moisture contaminant in low concentrations, typically under 1% by volume of the bulk gas, where high levels of purity are required in the product gas.
More recently, U.S. Pat. No. 7,087,102 B2 to Withers Jr. et al , discloses a process which advantageously employs the Skarstrom adsorption cascade (after U.S. Pat. No. 3,102,013 to Charles Skarstrom) under a pressure envelope up to 200 psig to purify mixtures of germanium hydrides, hydrogen and air impurities synthesized by the chemical reduction method, wherein water and carbon dioxide are selectively adsorbed in a first bed of molecular sieves selected from type 4A or other functionally similar adsorbent having an effective pore size greater than 4 angstroms, to produce a partially purified germane fluid. The partially purified germane fluid is then passed across a second molecular sieve selected from type 5A, 13X or other functionally similar adsorbents but having an effective pore size greater than 4 angstroms to adsorb heavier (than germanium hydride) germanium-containing compounds such as digermane (Ge2H6) and trigermane (Ge3H8), to obtain a hydrogen-enriched purified GeH4 fluid which is then separable by conventional methods. The disclosed method produces a germanium hydride product containing less than 1 vol. % of germanium-containing impurities.
Adsorption is an exothermic process and the heat liberated is similar in effect to the heat of condensation. Zeolitic molecular sieves have low specific heat capacities and the heat liberated in adsorbing water and carbon dioxide tends to warm up the adsorbent in direct proportion to the amount of adsorbate removed from the flowing gas stream. Sensible heating of zeolitic molecular sieves is known in the art to diminish separation efficiency at a fixed operating pressure. To compensate for the temperature-induced reduction in separation efficiency, higher operating pressures, greater volume of adsorbent per unit mass of gas to be purified, multiple zeolite adsorption stages or other effective combinations of these design parameters may be needed, to augment the overall adsorptive capacity at given operating conditions of impurity levels, gas flow rate, pressure and temperature. For practical reasons, a fluid purification column packed with zeolitic molecular sieves may itself be surrounded by external heating elements and shrouded beneath high capacity insulation to assist the regeneration sequence during which the column and its contents are heated to temperatures generally in excess of 180° C. but no more than circa 300° C. to drive off adsorbate withheld in the preceding purification sequence. In typical setups therefore, the flexibility to cool the adsorbent-containing vessel externally during gas purification is limited but can be incorporated at greater expense. The use of higher operating pressures to overcome temperature-induced deficiencies increases plant operating cost and escalates the potential hazard of toxic gas leaks. The use of greater adsorbent volume than is needed increases capital and operating costs. This detrimental effect of the heat of adsorption is unavoidable in the prior art wherein a significant portion of the moisture and carbon dioxide arising from the aqueous reactions is removed by adsorption from crude hydride gas. A further drawback of U.S. Pat. No. 7,087,102 is the adsorption of valuable digermane —Ge2H6 in the second adsorption step and its subsequent loss in an incinerating purge at temperatures up to 300° C. The safe disposal of desorbed digermane Ge2H6 in the prior art typified by U.S. Pat. No. 7,087,102 on a regular basis on commercial scale necessitates an elaborate scrubbing system to abate the hazard of vent streams released into the atmosphere, especially for germanium hydrides with a low exposure limit of 0.2 ppm in air. Alternatively, it may be required to chill the reactants, as taught in U.S. Pat. No. 4,668,502 to achieve lower yields of digermane. These challenges render the synthesis and purification of germanium hydrides to meet electronic specifications rather difficult and expensive. Not surprisingly, ultra pure germanium hydride in May 2007 costs upwards of $120 per gram when sourced in small quantities from mainstream catalogue retailers. An urgent need therefore exists in the art for the development of more cost-effective purification methods, which may be advantageously integrated to the high yielding synthesis method of Drake (1962).